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Many scientific investigations have focused on how space weather phenomena, taking place in the vicinity of the Earth, may influence different aspects of life on Earth and presumably human health itself. From 2005, the National and Kapodistrian University of Athens has established an important position in the field of these investigations by collaborating with various scientists and Institutes, both international and domestic, in different heliobiological projects. In this work, the Cosmic Ray Group of the National and Kapodistrian University of Athens has co-operated with the medical staff from different hospitals and clinics around the country so as to develop large records of medical data (heart rate) which covers a long time period. These data are analyzed in regard to physical activity, either on a daily basis or on different levels of geomagnetic disturbances and variations of the cosmic ray intensity using the ANalysis Of Variance (ANOVA) and the multiple linear regression analysis. Results suggest that space weather phenomena may be related to heart rate variability, i.e., heart rate is statistically significantly effected either by variations of cosmic rays intensity or geomagnetic activity.

A statistical study of interplanetary disturbances with different positions of the interplanetary magnetic field maximum that generate Forbush decreases is performed. It is shown that the most effective interplanetary disturbances are those with the position of the field maximum 6–15 h after the start of the event.

Cosmic ray cut-off rigidity tables and maps over the world concerning the epochs 2010, 2015 and the current one 2020 have been constructed. These maps display the effective cut-off rigidity in every five degrees in latitude and in longitude at the altitude of 20 km above the surface of the international reference ellipsoid. The values of the geomagnetic cut-off rigidity were calculated in every 5° in latitude and in every 15° in longitude applying the well-known method of particle trajectory calculations resulted from the theory of the particle motion in the Earth's magnetic field. The applied software employed the 12th Generation of the International Geomagnetic Reference Field (IGRF 12) and trajectories were calculated at 0.01 GV intervals in order to determine the vertical cut-off rigidity for each location. Beyond the use of the calculated cut-off rigidity values as a basic reference of charged particle access to different geographical locations during quiet and/or more intense geomagnetic periods, these results can be used for a long- term forecasting of the geomagnetic conditions variations.

In this study, the ionization rate in the atmosphere induced by solar and galactic cosmic rays is calculated for the region of Athens (Greece) during the time period from 1996 to 2019 covering the last two solar cycles 23 and 24. In order to compute the cosmic ray induced ionization, the corresponding model of the University of Oulu was used along with its new version which is extended to the upper atmosphere. This model has been applied to the entire atmosphere, i.e., from the atmospheric depth of 0.00 g/cm2 corresponding to the upper limit of the atmosphere (∼40 km), to that one of 1025 g/cm2 corresponding to the Earth’s surface. Furthermore, an application has been made as a function of rigidity and geomagnetic latitude, from 0.1 GV (∼90∘ polar regions) to 14.9 GV (∼0∘ equatorial regions). Specifically, we focus at the region of Athens that is a middle latitude one, located at 38∘N geographic latitude, and cosmic ray intensity is recorded by the sea level (260 m) neutron monitor station of the National and Kapodistrian University of Athens. Cosmic ray particles with a vertical cut-off rigidity of 8.5 GV are measured in real time and magnetospheric effects of the cosmic ray intensity with the maximum amplitude in the north hemisphere, are often observed. A comparison of the calculated cosmic ray induced ionization in this region with the ionization of polar and equatorial regions during the different phases of the solar cycles 23 and 24, is performed. A seasonal variation of this ionization during all the examined period is for first time observed. Obtained results are discussed in terms of Space Weather applications.

Assessing the radiation exposure of aviators and frequent flyers requires the study of the cosmic ray showers inside the Earth’s atmosphere. DYASTIMA / DYASTIMA-R is a Geant4 based software application, implemented by the Athens Cosmic Ray Group which allows the study of the evolving secondary particles cascades inside the atmosphere, as well as radiation dosimetry calculations (ambient dose equivalent rate) at different atmosphe - ric altitudes, geographic coordinates and magnetic cut-off rigidity. Results for various scenarios, as calculated by DYASTIMA/DYASTIMA-R, are provided as a federated product through the European Space Agency Space Situational Awareness of the Space Radiation Service Centre Network, while the DYASTIMA software is provi- ded through the Athens Neutron Monitor Station (A.Ne.Mo.S.) portal. Initial results for the assessment of the radiation exposure during the last Solar Cycles 23 and 24 are presented in this work, covering the most usual flying altitudes. The results indicate the dependence of the dose rate on the magnetic cut-off rigidity threshold, with higher dose rates at high geographic latitudes, as well as the anti-correlation of cosmic ray intensity with the solar activity, as higher dose rates are observed during solar minimum conditions.

Many previous studies have shown that before the beginning of a Forbush Decrease (FD) of the cosmic ray intensity, a precursor signal can be observed. All these surveys were focused on FDs that are associated with a sudden storm com- mencement (SSC). In this work we demonstrate that precursors could also be observed in events without a SSC that is determined by an abrupt increase of the interplanetary magnetic field. The type of precursory signals and their diversity among the events are the main purpose of this study. We try to figure out similarities and differences on the signals and the associated events from both categories in the last fifty years, from 1969 to 2019, using the same selection criteria of the under investigation FDs. Simultaneously the orientation of the upcoming solar disturbances in comparison to the way they configure the increase of the interplanetary magnetic field and create these signals are discussed.

In this work an improved approach of existing approximations on the coupling function between primary and ground-level cosmic-ray particles is presented. The proposed coupling function is analytically derived based on a formalism used in Quantum Field Theory calculations. It is upgraded compared to previous versions with the inclusion of a wider energy spectrum that is extended to lower energies, as well as an altitude correction factor, also derived analytically. The improved approximations are applied to two cases of Forbush decreases detected in March 2012 and September 2017. In the analytical procedure for the derivation of the primary cosmic-ray spectrum during these events, we also consider the energy spectrum exponent $\gamma $ to be varied with time. For the validation of the findings, we present a direct comparison between the primary spectrum and the amplitude values derived by the proposed method and the obtained time series of the cosmic-ray intensity at the rigidity of 10 GV obtained from the Global Survey Method. The two sets of results are found to be in very good agreement for both events as denoted by the Pearson correlation factors and slope values of their scatter plots. In such way we determine the validity and applicability of our method to Forbush decreases as well as to other cosmic-ray phenomena, thus introducing a new, alternative way of inferring the primary cosmic-ray intensity.

The study of space-weather effects and more specifically Forbush decreases of the cosmic-ray intensity depends on space and ground measurements. Very often Forbush decreases and geomagnetic storms are accompanied by pre-increases and/or pre-decreases manifested in cosmic-ray behavior, known as precursory signs. These cosmic-ray intensity variations do not coincide with the shock arrival but begin well before (up to 24 hours) the onset of the main event. In this study a group of large Forbush decreases with amplitude ≥ 4% was examined for precursors. According to the helio-longitude of the solar source, the events were separated into three categories: western ( $21^{\circ} \leq $ helio-longitude ≤ 60∘), eastern ( $-60^{\circ} \leq $ helio-longitude $\leq -21^{\circ}$ ), and central ( $-20^{\circ} \leq $ helio-longitude $\leq 20^{\circ}$ ). The selected events cover 1967 – 2017. The analysis of the Forbush decreases and the plotting of the asymptotic longitudinal cosmic-ray distribution diagrams were based on the “Global Survey Method” and the “Ring of Stations” method, respectively. Data on solar flares, solar-wind speed, interplanetary magnetic field, and geomagnetic indices (Kp and Dst) were also used. The results show the clear signs of precursors in a significant number of events.

In this investigation the different features and characteristics of Forbush decreases, with emphasis on large For- bush decreases (≥4%) and their association to solar sources, are being examined. According to the heliolongitude of the solar source, the events under study were separated into three subcategories: western (21º ≤ heliolongitude ≤ 60º), eastern (-60º ≤ heliolongitude ≤ -21º) and central (-20º ≤ heliolongitude ≤ 20º). The selected events cover the time period 1967 - 2017. The ‘Global Survey Method’ was used for analyzing the Forbush decreases, along with data on solar flares, solar wind speed, geomagnetic indices (Kp and Dst), and interplanetary magnetic field. In ad - dition, the superimposed epoch method was applied in order to plot the time profiles for the aforementioned group of events. This detailed analysis reveals interesting results concerning the features of cosmic ray decreases in re- gard to the heliolongitude of the solar sources. Moreover, it is also shown that large Forbush decreases, regardless of the heliolongitude of the solar source, are accompanied by increased geomagnetic activity and increased aniso- tropy, including anisotropy before the events, which can serve as a typical precursor of Forbush decreases.

One of the factors responsible for the wide variety of Forbush decreases is the different solar sources related to them. In this investigation the different features and characteristics of Forbush decreases, with emphasis on large Forbush decreases and their association with solar sources, are examined. Initially, a wider selection of events from the Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation of the Russian Academy of Sciences Forbush decreases database served as a starting point for this study, which was then narrowed down to a group of large Forbush decreases. According to the helio-longitude of the solar source, the events under study were separated into three subcategories: western ( $21^{\circ} \leq \mbox{helio-longitude} \leq 60^{\circ}$ ), eastern ( $-60^{\circ} \leq \mbox{helio-longitude} \leq -21^{\circ}$ ), and central ( $-20^{\circ} \leq \mbox{helio-longitude} \leq 20^{\circ}$ ). The selected events cover the period 1967 – 2017. The “Global Survey Method” was used for analyzing the aforementioned Forbush decreases, along with data on solar flares, solar-wind speed, geomagnetic indices (Kp and Dst), and interplanetary magnetic field. The superimposed epoch method was applied to display the temporal profiles for the selected events. This detailed analysis reveals interesting results concerning the features of cosmic-ray decreases in relation to the helio-longitude of the solar sources. Specifically, Forbush decreases related to central or eastern solar sources are more often observed, have a greater magnitude, and present a slower development than Forbush decreases related to western sources, which are rarer, have a smaller magnitude, and have a shorter lifespan. Nevertheless, regardless of the helio-longitude of the solar source, large Forbush decreases are accompanied by increased geomagnetic activity and increased anisotropy, including anisotropy before the events, which can serve as a typical precursor of Forbush decreases.

The Forbush decreases of the cosmic ray intensity observed on 24 December 2014 and on 8 September 2017 were chosen for cosmic ray spectral analysis. At first an analytical study of the solar and geomagnetic parameters of these events was carried out due to the fact that both are typical cosmic ray events. Hourly cosmic ray data of the neutron monitor stations obtained from the high-resolution neutron monitor database were used for calculating the cosmic ray density and anisotropy variations. Following the method of the coupling coefficients, the galactic cosmic ray spectral index was calculated using the technique of Wawrzynczak and Alania (Adv. Space Res. 45, 622, 2010). A newly presented yield function by Mishev, Usoskin, and Kovaltsov (J. Geophys. Res. 118, 2783, 2013) including a geometrical correction factor, already used in the spectral analysis of the cosmic ray ground level enhancements, was applied for the first time to the case of Forbush decreases. A comparison of these results during the events is performed by using two other coupling functions: the function presented in the work of Clem and Dorman (Space Sci. Rev. 93, 335, 2000) and the one in the work of Belov and Struminsky (Proc. 25th Int. Cosmic Ray Conf. 1, 201, 1997). The latter includes an extension for neutron monitor stations with rigidity $1~\mbox{GV} < R < 2.78~\mbox{GV}$ . The obtained spectral index and the calculated cosmic ray intensity in the heliosphere during the two Forbush decreases after the coupling by these three functions are presented and discussed.

The study of the particle showers created inside the Earth’s atmosphere due to interactions of cosmic rays of solar and galactic origin is of great importance for the determination of the radiation impact on technological and biological systems. DYASTIMA is a Geant4-based software application that simulates the evolution of secondary particle cascades inside the atmosphere of Earth. DYASTIMA-R is a new feature especially created for assessing the exposure of flight-personnel and frequent flyers to cosmic radiation by performing calculations of radiobiological quantities, such as dose and equivalent dose rates for several air-flight scenarios. In this work, the validation of DYASTIMA/DYASTIMA-R, according to internationally accepted ICRP and ICRU standards, is discussed. Initial results for radiobiological quantities for several air-flight scenarios are also included. The results for specific scenarios calculated by DYASTIMA/DYASTIMA-R are provided as a federated product through the European Space Agency Space Situational Awareness Space Weather Service Centre Network.

On 10 September 2017, a ground level enhancement (GLE) of cosmic ray intensity, identified as GLE72, was recorded by several stations of the worldwide neutron monitor network provided by the high‐resolution Neutron Monitor Database. The solar proton event that resulted in this GLE was associated with active region AR2673, which produced an X8.2 flare on the solar west limb. Protons were measured by the GOES satellites with energies above 10, 50, and 100 MeV, while particles at higher energies above 500 MeV were registered by ground‐based neutron monitors. This GLE event was successfully detected in real time by the GLE Alert plus System of the Athens Neutron Monitor Station (A.Ne.Mo.S.). In this work an overview of the GLE72 event is given, and a detailed analysis of the evolution of the GLE Alert signal issued by the GLE Alert plus System as well as a postevent summary are presented.

Interplanetary coronal mass ejections (ICMEs) are believed to be the most common and important drivers of the strongest geomagnetic storms. In this work, the geoeffective characteristics of the ICMEs occurred during the last solar cycles 23 and 24 (years 1996–2017) have been studied in detail. The maximum velocity $V$max, either of the ICME’s Sheath region or of the ICME itself, the mean velocity of the ICME, the minimum value of the southward component of the Interplanetary magnetic field Bs and the $y$-component of the solar wind convective electric field $E= -V \times B$ observed at L1 point during the pass of the ICME, were used. It was found that, in accordance to past similar studies, the most dominant characteristic of ICMEs in the generation of geomagnetic storms is the Bs component along with the Ey parameter, while the maximum velocity seems to be of less importance. Nevertheless the maximum speed is an good forecasting factor due to the fact that it is much easier to estimate the velocity of an ICME-structure many hours before it arrives at Earth compared with the observations of Bs and Ey that can only be done, for the time being, at the L1 point. That means we can use the velocity of ICME-structure to forecast the possible generation and magnitude of the geomagnetic storms. From a comparison of the ICME-generated geomagnetic storms with the total number of geomagnetic storms generated during the last two solar cycles, it seems that approximately half of the ICMEs (49% for Dst index and 53% for Kp index) produced geomagnetic storms during the solar cycles 23 and 24. Moreover the velocities of ICMEs are more in accordance with the rising and maximum phases of solar cycles 23 and 24 than the geomagnetic activity (storms) are, as well as during the first stages of the declining phases of these cycles, especially during solar cycle 23.

In spite of the fact that the current Solar Cycle 24 is close to its end now, it is a less active Solar Cycle, during its time period (2008 – 2016) and a lot of Forbush decreases of cosmic ray intensity with rigidity 10 GV and amplitude greater than 2% were recorded by the ground-based neutron monitors. Among these events, the ones associated with sudden geomagnetic storm commencements (SSCs) and presenting a first harmonic of cosmic ray anisotropy greater than 0.8% were examined. Cosmic ray data recorded at the neutron monitor stations were obtained from the European high resolution neutron monitor database, while the Forbush decreases, accompanied by their characteristics were accessed from the updated Database of the Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation (IZMIRAN). Solar, interplanetary and geomagnetic characteristic parameters of each event separately have been studied in detail and analyzed. It was shown through the usage of the “ring of neutron monitor stations” method that, in some cases, precursory signals before the main phase of the event appeared. After an extended study of the Forbush decreases precursors during the examined period, the appearance of pre-decreases and/or pre-increases of the cosmic ray intensity before the beginning of the events, acting as precursory signals, were identified in almost half of the cases studied. In combination with other parameters, their common features are discussed, with the purpose of monitoring and possibly forecasting the space-weather conditions.

In this work, forecasting results of the daily Ap geomagnetic index by different Space Weather Prediction Centers (SWPCs) covering the time period from October 2014 up to July 2020, are considered. A three-day forecast of this index obtained from the space weather prediction centers, is analyzed. Standard forecast verification measures, descriptive statistics with correlation coefficients, and error analysis between forecasts and observations have been performed to evaluate the quality or the skill of the predictions. In particular, an error analysis using fit performance metrics, such as the mean average error (MAE) and the root mean squared error (RMSE) among others, as well as the threshold performance metrics, such as the Hansen-Kuipers, Gilbert and Heidke skill scores, the probability of detection (POD), the probability of false detection (POFD) and the area under the curve in receiving the operating characteristic (ROC) plot among others, are calculated. For the Ap geomagnetic index predictions during Day-0, the Pearson's correlation coefficient is ranging between the values of 0.57 and 0.79, while during the Day-2 it is decreased to the range from 0.37 to 0.44. In conclusion, the majority of the used SWPCs perform quite accurately on the conditions of Space Weather in active as well as in quiet periods showing a reliable effort for predicting geomagnetic storms.

The biological human system is probably affected by the solar and geomagnetic disturbances as well as the cosmic ray variations. In this work, the relation between the solar activity and cosmic ray variations and the cardiac arrhythmias over the time period 1997–2009 covering the solar cycle 23, is studied. The used medical data set refers to 4741 patients with cardiac arrhythmias and 2548 of whom were diagnosed with atrial fibrillation, obtained from the 2nd Cardiological Clinic of the General Hospital of Nicaea, Piraeus, in Greece. The smoothing method on a 365-day basis and the Pearson r-coefficient were used in order to compare these records with the number of sunspots, flares, solar proton events, coronal mass ejections and cosmic ray intensity. Applying a moving correlation function to ±1500 days, it is suggested that a change of the correlation sign between the medical data and each one of the above parameters occurs during a time interval of about 2–3 years. This interval corresponds to the time span of the polarity reversal of the solar magnetic field of this solar cycle, which always takes place around the solar cycle maximum. After then a correlation analysis was carried out corresponding to the rise (1997–2001) and the decay (2002–2009) phases of the solar cycle 23. It is noticeable that the polarity reversal of the solar magnetic field coincides with the period where the sign of the correlation between the incidence of arrhythmias and the occurrence number of the solar eruptive events and the cosmic ray intensity, changes sign. The results are comparable with those obtained from the previous solar cycle 22 based on medical data from another country.

The diurnal variation of cosmic ray intensity, based on the records of two neutron monitor stations at Athens (Greece) and Oulu (Finland) for the time period 2001 to 2014, is studied. This period covers the maximum and the descending phase of the solar cycle 23, the minimum of the solar cycles 23/24 and the ascending phase of the solar cycle 24.These two stations differ in their geographic latitude and magnetic threshold rigidity. The amplitude and phase of the diurnal anisotropy vectors have been calculated on annual and monthly basis. From our analysis it is resulted that there is a different behaviour in the characteristics of the diurnal anisotropy during the different phases of the solar cycle, depended on the solar magnetic field polarity, but also during extreme events of solar activity, such as Ground Level Enhancements and cosmic ray events, such as Forbush decreases and magnetospheric events. These results may be useful to Space Weather forecasting and especially to Biomagnetic studies.

We attempt the derivation of a new coupling function between ground level and primary cosmic rays for the case of Forbush decreases. The calculations for the new function are based on basic Quantum Field Theory theoretical tools, something that has not been attempted till now in other widely used cosmic ray coupling functions. The use of Quantum Field Theory calculations in cosmic rays events in general, is expected to be a suitable frame of work since it describes well the high energy particle interactions which result in variations to the total number of the particles involved through annihilation or creation of particles. The newly computed function is tested to the case of two events of Forbush decreases, February 2011 and March 2012, using data from the high resolution neutron monitor database. Results for the primary particle intensity values obtained from this function are compared directly to the corresponding ones from the use of the Dorman's widely accepted coupling function. The two sets are discussed in detail in order to deduce the possible suitability of Quantum Field Theory tools to cosmic ray events.

We present an overview of the ground-level enhancement (GLE 72) of the cosmic-ray intensity associated with the recent powerful solar flare SOL2017-09-10 (X-ray class X8.9) based on the available neutron monitor (NM) network observations and on data from the satellite GOES 13. The maximum increase at high-latitude near-sea-level NMs was ${\approx }\,6\,\mbox{--}\,7\%$ (2-min averages), greater with better time resolution. A scatter plot of the maximum increase of the GLE versus solar energetic-particle (SEP, proton) flux ${>}\,100~\mbox{MeV}$ shows one of the softest spectra among GLEs relative to the SEP fluxes. However, at two high-mountain middle-latitude NMs the increase was ${\approx }\,1\%$ , indicating the possibility of proton acceleration up to 6 GeV. Among the analyzed NM data the Fort Smith (FSMT) NM shows the earliest and the rather high increase between 16:06 – 16:08 UT. This indicates an anisotropy in the first phase of the GLE event. We calculate the acceptance cones of several NM stations at high latitudes and contours of pitch angles corresponding to the interplanetary magnetic field (IMF). When employing the available data we find that pion-decay $\gamma $ -ray emission onset is in accordance with the time of the main flare energy release. The observed time interval of the impulsive burst of ${>}\,100~\mbox{MeV}$ $\gamma $ -ray emission probably corresponds to the time of a turbulent current sheet creation. The observed location of the impulsive burst pion-decay emission source coincides with the active region and the cusp-shaped structure. It seems that models assuming sub-relativistic proton production beginning in a turbulent reconnecting current sheet are consistent with the observations. If these particles were released from the Sun during a type III emission with a pion-decay maximum at $16{:}00{:}30\pm 30~\mbox{UT}$ , we get a plausible path length equal to $1.5\pm 0.3~\mbox{AU}$ of the particles responsible for the onset of the SEP event and GLE. The time lag of GLE 72 corresponds to the most probable interval of the time difference between GLE onset and main flare energy release. Although other scenarios are not excluded we attribute the protons that create the pion-decay emission and the protons responsible for the GLE and SEP event onset to a general population of accelerated particles.

Spectral analysis of solar and geomagnetic parameters as well as of cosmic-ray intensity was performed aiming to identify possible new periodicities and confirm the well-known ones. Specifically, short-, mid-, and long-term periodicities of these parameters such as sunspot number, Bz-component of the interplanetary magnetic field, geomagnetic Ap index, and cosmic-ray intensity over the time period 1965 – 2018, covering five solar cycles from Cycles 20 to 24, are presented. For this purpose, two different techniques, fast Fourier transformation and wavelet analysis, have been used in order to ensure accuracy in the frequency values and also their localization in the time series. The periodicities resulting from our comprehensive study, including the well-known 11-year and 27-day periods, the harmonics of the 5.5-year and of the 6-, 9-, and 13.9-day periods, respectively, and the ≈ 1.3-year and 1.7-year periods, were found in all of the above parameters except for the Bz-component of the interplanetary magnetic field. New periodicities such as the ≈ 10-month period for sunspot number and cosmic-ray intensity and the ≈ 3-year period for sunspot number, Ap index, and cosmic-ray intensity, were also determined. Furthermore, the newly introduced splitting of the 27-day periodicity into two adjacent peaks was confirmed in the Fourier spectra of the interplanetary magnetic field and the geomagnetic Ap index. It was concluded that several common periodicities appear in solar activity: the Ap index, and the cosmic-ray intensity. This result, in association with the fact that the spectral behavior of geomagnetic-activity parameters, provides invaluable information about the physical processes involved, and indicates that the Ap index might be used as a suitable index for space-weather forecasting.

The relative importance of the two most likely modes of input energy dissipation during the substorm of 8 May 1986, with an onset at 12:15 UT (CDAW 9E event), is examined here. The combination of data from the interplanetary medium, the magnetotail and the ground allowed us, first of all, to establish the sequence of phenomena which compose this substorm. In order to calculate the magnetospheric energetics we have improved the Akasofu model, by adding two more terms for the total magnetospheric output energy. The first one represents the energy consumed for the substorm current wedge transformation, supplied by the asymmetric ring current. This was found to be 39% of the solar wind energy entering the magnetosphere from the start of the growth phase up to the end of the expansion phase. The second term represents the energy stored in the tail or returned to the solar wind. Our results suggest that the substorm leaves the magnetosphere in a lower energy state, since, according to our calculations, 23% of the energy that entered the magnetosphere during the whole disturbance was returned back to the solar wind. Finally, it is interesting to note that during the growth phase the driven system grow considerably, consuming 36% of the solar wind energy which entered the magnetosphere during this early phase of the substorm.

An updated catalog is created of 303 well-defined high-speed solar wind streams that occurred in the time period 2009 – 2016. These streams are identified from solar and interplanetary measurements obtained from the OMNIWeb database as well as from the Solar and Heliospheric Observatory (SOHO) database. This time interval covers the deep minimum observed between the last two Solar Cycles 23 and 24, as well as the ascending, the maximum, and part of the descending phases of the current Solar Cycle 24. The main properties of solar-wind high-speed streams, such as their maximum velocity, their duration, and their possible sources are analyzed in detail. We discuss the relative importance of all those parameters of high-speed solar wind streams and especially of their sources in terms of the different phases of the current cycle. We carry out a comparison between the characteristic parameters of high-speed solar wind streams in the present solar cycle with those of previous solar cycles to understand the dependence of their long-term variation on the cycle phase. Moreover, the present study investigates the varied phenomenology related to the magnetic interactions between these streams and the Earth’s magnetosphere. These interactions can initiate geomagnetic disturbances resulting in geomagnetic storms at Earth that may have impact on technology and endanger human activity and health.

Although the current Solar Cycle 24 is characterized by low solar activity, an intense geomagnetic storm (G4) was recorded in June 2015. It was a complex phenomenon that began on 22 June 2015 as the result of intense solar activity, accompanied by several flares and coronal mass ejections that interacted with the Earth’s magnetic field. A Forbush decrease was also recorded at the neutron monitors of the worldwide network, with an amplitude of 8.4%, and in its recovery phase, a second Forbush decrease followed, with an amplitude of 4.0% for cosmic rays of 10 GV obtained with the global survey method. The Dst index reached a minimum value of −204 nT that was detected on 23 June 2015 at 05:00 – 06:00 UT, while the Kp index reached the value eight. For our analysis, we used hourly cosmic-ray intensity data recorded by polar, mid-, and high-latitude neutron monitor stations obtained from the High Resolution Neutron Monitor Database. The cosmic-ray anisotropy variation at the ecliptic plane was also estimated and was found to be highly complex. We study and discuss the unusual and complex cosmic-ray and geomagnetic response to these solar events.

The global survey method (GSM) technique unites simultaneous ground-level observations of cosmic rays in different locations and allows us to obtain the main characteristics of cosmic-ray variations outside of the atmosphere and magnetosphere of Earth. This technique has been developed and applied in numerous studies over many years by the Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation (IZMIRAN). We here describe the IZMIRAN version of the GSM in detail. With this technique, the hourly data of the world-wide neutron-monitor network from July 1957 until December 2016 were processed, and further processing is enabled upon the receipt of new data. The result is a database of homogeneous and continuous hourly characteristics of the density variations (an isotropic part of the intensity) and the 3D vector of the cosmic-ray anisotropy. It includes all of the effects that could be identified in galactic cosmic-ray variations that were caused by large-scale disturbances of the interplanetary medium in more than 50 years. These results in turn became the basis for a database on Forbush effects and interplanetary disturbances. This database allows correlating various space-environment parameters (the characteristics of the Sun, the solar wind, et cetera) with cosmic-ray parameters and studying their interrelations. We also present features of the coupling coefficients for different neutron monitors that enable us to make a connection from ground-level measurements to primary cosmic-ray variations outside the atmosphere and the magnetosphere. We discuss the strengths and weaknesses of the current version of the GSM as well as further possible developments and improvements. The method developed allows us to minimize the problems of the neutron-monitor network, which are typical for experimental physics, and to considerably enhance its advantages.

We study the Forbush decreases in cosmic-ray intensity from January 2008 to December 2013, covering the minimum between Solar Cycles 23 and 24 and the ascending phase of Cycle 24. We performed a statistical analysis of 617 events and concentrated on three of the most important ones. We used the IZMIRAN database of Forbush effects obtained by processing the data of the worldwide neutron monitor network using the global survey method. The first event occurred on 18 February 2011 with a ${\sim}\,5~\%$ decrease of cosmic rays with 10 GV rigidity, the second on 8 March 2012 with an amplitude of ${\sim}\,12~\%$ , and the third on 14 July 2012 with an amplitude of ${\sim}\,6~\%$ . For these three events, we also studied the events that occurred on the Sun and the way that these affected the interplanetary space, and finally provoked the decreases of the galactic cosmic rays near Earth. We found that each neutron monitor records these decreases, which depend on the cut-off rigidity of the station. We carried out a statistical analysis of the amplitude of the cosmic-ray decreases with solar and geomagnetic parameters.

Two independent methods for estimating basic parameters of the solar cycle are presented. The first of them, the ascending-descending triangle method, is based on a previous work by Tritakis (Astrophys. Space Sci. 82, 463, 1982), which described how the fundamental parameters of a certain solar cycle could be predicted from the shape of the previous one. The relation between the two cycles before and after a specific 11-year solar cycle is tighter than between the two cycles belonging to the same 22-year solar cycle (even-odd cycle). The second is the MinimaxX method, which uses a significant relation in the international sunspot number between the maximum value of a solar cycle and its value 2.5 or 3 years (depending on the enumeration of the even or odd cycle) before the preceding minimum. The tests applied to Cycles 12 to 24 indicate that both methods can estimate the peak of the 11-year solar radio flux at a high confidence level. The data used in this study are the 10.7 cm solar radio flux since 1947, which have been extrapolated back to 1848 from the strong correlation between the monthly international sunspot numbers and the adjusted values of the 10.7 cm radio flux.

We investigate the characteristics of Galactic cosmic rays in events associated with magnetic clouds that reach Earth. A mathematical model, capable of describing the distribution of the cosmic-ray density in a magnetic cloud is considered. We show that in most cases the behavior of the cosmic-ray density within magnetic clouds at 1 AU can be described accurately by a parabolic function of the distance to the center of the magnetic cloud measured in gyroradii. As expected, the majority of magnetic clouds modulate cosmic rays, resulting in a reduction of their density. However, there is a group of events (about one fifth of the total sample) in which the density of cosmic rays in a magnetic cloud increases. Furthermore, the extremum (a minimum or a maximum) of the cosmic-ray density is found closer to the cloud center and not at its edges. We consider a number of the factors contributing to the model and estimate the effect of each factor.

During the burst of solar activity in March 2012, close to the maximum of solar cycle 24, a number of X-class and M-class flares and halo CMEs with velocity up to $2684~\mbox{km}/\mbox{s}$ were recorded. During a relatively short period (7–21 March 2012) two Forbush decreases were registered in the ground-level neutron monitor data. In this work, after a short description of the solar and geomagnetic background of these Forbush decreases, we deduce the cosmic ray density and anisotropy variations based on the daily cosmic ray data of the neutron monitor network ( http://www.nmdb.eu ; http://cosray.phys.uoa.gr ). Applying to our data two different coupling functions methods, the spectral index of these Forbush decreases was calculated following the technique of Wawrzynczak and Alania (Adv. Space Res. 45:622–631, 2010). We pointed out that the estimated values of the spectral index $\gamma$ of these events are almost similar for both cases following the fluctuation of the Forbush decrease. The study and the calculation of the cosmic ray spectrum during such cosmic ray events are very important for Space Weather applications.

In a previous work (Paouris and Mavromichalaki in Solar Phys. 292, 30, 2017), we presented a total of 266 interplanetary coronal mass ejections (ICMEs) with as much information as possible. We developed a new empirical model for estimating the acceleration of these events in the interplanetary medium from this analysis. In this work, we present a new approach on the effective acceleration model (EAM) for predicting the arrival time of the shock that preceds a CME, using data of a total of 214 ICMEs. For the first time, the projection effects of the linear speed of CMEs are taken into account in this empirical model, which significantly improves the prediction of the arrival time of the shock. In particular, the mean value of the time difference between the observed time of the shock and the predicted time was equal to +3.03 hours with a mean absolute error (MAE) of 18.58 hours and a root mean squared error (RMSE) of 22.47 hours. After the improvement of this model, the mean value of the time difference is decreased to −0.28 hours with an MAE of 17.65 hours and an RMSE of 21.55 hours. This improved version was applied to a set of three recent Earth-directed CMEs reported in May, June, and July of 2017, and we compare our results with the values predicted by other related models.

During the last years a possible link between different levels of solar and geomagnetic disturbances and human physiological parameters is suggested by several published studies. In this work the examination of the potential association between heart rate variations and specific space weather activities was performed. A total of 482 individuals treated at Hippocratio General Hospital in Athens, the Cardiology clinics of Nikaia General Hospital in Piraeus and the Heraklion University Hospital in Crete, Greece, were assessed from July 2011 to April 2013. The heart rate of the individuals was recorded by a Holter monitor on a n hourly basis, while the hourly variations of the cosmic ray intensity measured by the Neutron Monitor Station of the Athens University and of the geomagnetic index Dst provided by the Kyoto Observatory were used. The ANalysis Of VAriance (ANOVA) and the Multiple Linear Regression analysis were used for analysis of these data. A statistically significant effect of both cosmic rays and geomagnetic activity on heart rate was observed, which may indicate that changes in space weather could be linked to heart rate variations.

Galactic cosmic rays and solar energetic particles with sufficient rigidity to penetrate the geomagnetic field, enter the Earth’s atmosphere and interact with the electrons and the nuclei of its atoms and molecules. From the interactions with the nuclei, cascades of secondary particles are produced that can be detected by ground-based detectors such as neutron monitors and muon counters. The theoretical study of the details of the atmospheric showers is of great importance, since many applications, such as the dosimetry for the aviation crews, are based on it. In this work, a new application which can be used in order to study the showers of the secondary particles in the atmosphere is presented. This application is based on the Monte Carlo simulation techniques, performed by using the well-known Geant4 toolkit. We present a thorough analysis of the simulation’s critical points, including a description of the procedure applied in order to model the atmosphere and the geomagnetic field. Representative results obtained by the application are presented and future plans for the project are discussed.

Whenever a significant intensity increase is being recorded by at least three neutron monitor stations in real-time mode, a ground level enhancement (GLE) event is marked and an automated alert is issued. Although, the physical concept of the algorithm is solid and has efficiently worked in a number of cases, the availability of real-time data is still an open issue and makes timely GLE alerts quite challenging. In this work we present the optimization of the GLE alert that has been set into operation since 2006 at the Athens Neutron Monitor Station. This upgrade has led to GLE Alert Plus, which is currently based upon the Neutron Monitor Database (NMDB). We have determined the critical values per station allowing us to issue reliable GLE alerts close to the initiation of the event while at the same time we keep the false alert rate at low levels. Furthermore, we have managed to treat the problem of data availability, introducing the Go-Back-N algorithm. A total of 13 GLE events have been marked from January 2000 to December 2012. GLE Alert Plus issued an alert for 12 events. These alert times are compared to the alert times of GOES Space Weather Prediction Center and Solar Energetic Particle forecaster of the University of Malaga (UMASEP). In all cases GLE Alert Plus precedes the GOES alert by ≈8–52 min. The comparison with UMASEP demonstrated a remarkably good agreement. Real-time GLE alerts by GLE Alert Plus may be retrieved by http://cosray.phys.uoa.gr/gle_alert_plus.html, http://www.nmdb.eu, and http://swe.ssa.esa.int/web/guest/space-radiation. An automated GLE alert email notification system is also available to interested users.

Coronal mass ejections (CMEs) and their interplanetary counterparts (interplanetary coronal mass ejections, ICMEs) are responsible for large solar energetic particle events and severe geomagnetic storms. They can modulate the intensity of Galactic cosmic rays, resulting in non-recurrent Forbush decreases (FDs). We investigate the connection between CME manifestations and FDs. We used specially processed data from the worldwide neutron monitor network to pinpoint the characteristics of the recorded FDs together with CME-related data from the detailed online catalog based upon the Solar and Heliospheric Observatory (SOHO)/Large Angle and Spectrometric Coronagraph (LASCO) data. We report on the correlations of the FD magnitude to the CME initial speed, the ICME transit speed, and the maximum solar wind speed. Comparisons between the features of CMEs (mass, width, velocity) and the characteristics of FDs are also discussed. FD features for halo, partial halo, and non-halo CMEs are presented and discussed.

Space radiation mainly consists of trapped particles inside the Earth’s magnetosphere, galactic and solar cosmic rays. Cosmic rays propagating in the interplanetary medium, reach the top of the Earth’s atmosphere and collide with the molecules of the atmospheric layers creating showers of secondary particles that can be recorded by ground-based neutron monitors or muon detectors. Due to these cascades, the radiation environment in various atmospheric altitudes is entirely different than the one experienced on the Earth’s surface. Space radiation is ionizing (trapped particles, galactic and solar cosmic rays) as well as non-ionizing (ultra-violet radiation). It is known that ionizing radiation is very dangerous for all biological systems, causing a variety of acute and chronic effects. The determination of the occupational exposure of aircrews to space radiation is of great importance. DYnamic Atmospheric Shower Tracking Interactive Model Application (DYASTIMA), as well as its extension DYASTIMA-R, is a standalone application, based on the toolkit Geant4, that simulates the propagation of cosmic radiation into the atmosphere. DYASTIMA provides all the necessary information about the study of the cascade in different atmospheric altitudes, while DYASTIMA-R, as a dosimetry application, calculates radiobiological quantities, such as dose rate and equivalent dose rate for the determination of the exposure, based on the output provided by DYASTIMA. These quantities are calculated for solar minimum and solar maximum conditions. DYASTIMA can be accessed through the portals ofthe European Space Agency (ESA) (http://swe.ssa.esa.int/spaceradiation) and the Athens Neutron Monitor Station (A.Ne.Mo.S) (http://cosray.phys.uoa.gr/index.php/dyastima).

The neutron monitors are the ground based detectors that continuously measure the flow of the cosmic rays that reach the earth′s surface. The measurements of the neutron monitors are of great importance for the scientific community since they contribute to the study of several scientific fields, such as the solar activity and the prediction of the space weather. For this reason, most of the neutron monitors worldwide are organized in a network, in order for their measurements to be easily accessible. The correct evaluation of the measurements and their connection with the physical quantities of the cosmic rays require the knowledge of the interactions and the detection procedure that take place inside the neutron monitor. In this work a quantitative study of the 6NM-64 behavior is presented based on Monte Carlo simulations by using the well known Geant4 simulations toolkit. The study focuses on the detection efficiency of the neutron monitor, both in sections and as a whole for the different particle species, on its dependence on the incident direction of the particles and on the secondary neutrons produced inside the neutron monitor.